The purpose is to be realistic. E.g. some trades that are open will give an overall of X amount of drawdown so the next trade with exactly the same lot size will initiate a spread which will decrease the amount of our equiity and a possible drawdown which it will decrease it further, so what a trader should do in these realistic circumstances??? Stick to the plan! Cumulatively, with the passing of days, some will end up in wins increasign the equity while others will end to losses which will decrease. Subsequently, risk management and correct exit points are key significant factors for trading success. We just do not want for trades to create an open loss of more than 5% daily and 10% in consecutive days, while at the same time we are chasing as much as we can according to our skills, mood, news, perception, normal job, free time, initial investemetn etc etc etc
Food for thought...
P.S. Every trade has been taken with 1 lot size - except one or two which have been executed as twin trading with half and half.